1. Field of the Invention
The present invention relates to an optical member and a method for producing the optical member.
2. Description of the Related Art
There have been proposed a lot of films having a fine concave-convex surface and a great number of these films are suitable for use as antireflection materials. For example, there has been proposed an optical member including a substrate and an antireflection film which has fine concavity and convexity formed on and supported by a thin film layer formed on the substrate, the fine concavity and convexity being formed of a plate-like crystal made mainly from alumina and the thin film layer being composed of an oxide including at least one selected from zirconia, silica, titania, and zinc oxide (see Japanese Patent (JP-B) No. 4182236).
However, the disclosed optical member employs a liquid-phase film forming method (sol-gel method) as a method for forming the fine concavity and convexity, and such method is incompatible with a method for laminating the oxide thin film conventionally used for a lens. Moreover, the number of steps increases because a step of forming the thin film on the substrate is added. Furthermore, there are problems in that film adhesion and coating properties on a structure on which liquid readily collects, such as a concavity, are poor.
Moreover, Japanese Patent Application Publication (JP-B) No. 61-48124 discloses a technique of forming a fine concave-convex structure on a surface by vacuum vapor deposition of metal aluminum on a substrate and then subjecting the deposited metal aluminum to a hydrothermal treatment. However, Japanese Patent Application Publication (JP-B) No. 61-48124 does not disclose techniques of employing a lens having a curvature as the substrate and forming a dielectric layer between the substrate and the fine concave-convex structure.
Therefore, there have been demands for an optical member which has an antireflection effect on its entire surface or a certain part of its surface even when a substrate having an at least partially curved surface is used; and a method for producing the optical member, whereby a thickness of a film can be easily controlled, high film adhesion is achieved, a film can be easily formed on a structure which is incompatible with a liquid phase reaction such as a substrate having an at least partially curved surface, and at least one of aluminum and alumina can be deposited after forming a dielectric layer by gas-phase deposition without breaking vacuum.